FEATURE: TL4 & category moderators can merge posts (#12843)

This gives TL4 users and category moderators the ability to merge posts in topics they have moderation privileges.

lib/post_merger.rb

@@ -9,12 +9,13 @@ class PostMerger
   end
 
   def merge
-    return unless ensure_at_least_two_posts
+    return if @posts.count < 2
+
     ensure_same_topic!
     ensure_same_user!
 
     guardian = Guardian.new(@user)
-    ensure_staff_user!(guardian)
+    ensure_can_merge!(guardian)
 
     posts = @posts.sort_by do |post|
       guardian.ensure_can_delete!(post)
@@ -38,29 +39,25 @@ class PostMerger
 
   private
 
-  def ensure_at_least_two_posts
-    @posts.count >= 2
-  end
-
   def ensure_same_topic!
-    unless @posts.map(&:topic_id).uniq.length == 1
+    if @posts.map(&:topic_id).uniq.size != 1
       raise CannotMergeError.new(I18n.t("merge_posts.errors.different_topics"))
     end
   end
 
   def ensure_same_user!
-    unless @posts.map(&:user_id).uniq.length == 1
+    if @posts.map(&:user_id).uniq.size != 1
       raise CannotMergeError.new(I18n.t("merge_posts.errors.different_users"))
     end
   end
 
-  def ensure_staff_user!(guardian)
-    raise Discourse::InvalidAccess unless guardian.is_staff?
+  def ensure_can_merge!(guardian)
+    raise Discourse::InvalidAccess unless guardian.can_moderate_topic?(@posts[0].topic)
   end
 
   def ensure_max_post_length!(raw)
     value = StrippedLengthValidator.get_sanitized_value(raw)
-    if value.length > SiteSetting.max_post_length
+    if value.size > SiteSetting.max_post_length
       raise CannotMergeError.new(I18n.t("merge_posts.errors.max_post_length"))
     end
   end